1. Field of the Invention
The present invention relates to a light source device for generating extreme ultra violet (EUV) light by applying a laser beam to a target.
2. Description of Related Art
As semiconductor processes become finer, the photolithography has been making rapid progress to finer fabrication, and, in the next generation, microfabrication of a pattern having a width of 100 nm to 70 nm, further, microfabrication of a pattern having a width of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of a pattern having a width of 50 nm or less, the development of exposure equipment with a combination of an EUV light source for generating light having a wavelength of about 13 nm and a reduced projection cataoptric system is expected.
As the EUV light source, there are three kinds of an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably increased, light emission of only the necessary waveband can be performed by selecting the target material, and an extremely large collection solid angle of 2π sterad can be ensured because it is a point source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography in which power of several tens of watts are required.
In the LPP light source, in the case where a solid material is used as a target to which a laser beam is applied for generating plasma, the heat generated by application of the laser beam is conducted to the periphery of the laser beam applied region when the laser beam application region turns into a plasma state, and the solid material is melted on the periphery thereof. The melted solid material becomes debris having a diameter of several micrometers or more, which is emitted in large quantity and causes damage to the collection mirror (specifically, to the mirror coating) and reduces the reflectance thereof. Also, in the case where a liquid material is used as the target, the flying debris causes damage to the collection mirror. On the other hand, in the case where a gas is used as the target, although debris is reduced, the conversion efficiency from the power supplied to the driving laser to the power of the EUV light is reduced.
As a related technology, Japanese Patent Publication JP-B-3433151 discloses a laser plasma X-ray source in which damage to an optical mirror due to the generated debris is prevented and the collection efficiency of X-ray is improved. According to the document, the X-ray source includes a magnetic field applying device for applying a magnetic field in a direction perpendicular to an injection direction of a target. Assuming that the traveling direction of the debris before deflected by the magnetic field is the injection direction of the target, the damage to the optical mirror can be pretended by locating the optical mirror in a direction in which ionic state debris deflected by the magnetic field do not fly.
Further, Japanese Patent Publication JP-B-2552433 discloses a removing method and device capable of radically removing debris generated from a solid target in a relative simple manner. According to the document, electric charges are provided by ultraviolet light to neutral fine particles produced with X-rays from plasma on the surface of a target material, an electromagnetic field in which an electric field and a magnetic field are mutually perpendicular is generated by a pair of mesh-form electrodes arranged along the path of X-ray and an electromagnetic disposed between the pair of electrodes, and the charged fine particles are passed through the electromagnetic field such that the orbit of the charged fine particles can be bent and eliminated to the outside of the X-ray path. Thereby, an X-ray optical element provided in the X-ray path can be protected.